It is recalled that the term "regulating" means maintaining the physical magnitude to be regulated at a predetermined constant value.
The apparatus of the present invention is suitable for being applied to two different categories of equipment.
A first category concerns regulating the internal pressure of any moving, ventilated vehicle subjected to variations in external pressure, e.g. consequent upon rapid changes in its aerodynamic field or upon variations in altitude.
By way of example, very high speed trains may be mentioned.
In this first category of equipment, unregulated variations in pressure would be physiologically unpleasant for the user.
A second category generally concerns any ventilated volume in communication with a varying external pressure.
In a vehicle or a volume in which internal air is renewed by air-conditioning or ventilation using external air, the internal pressure is close to the external pressure.
Variations in external pressure are passed on almost instantaneously to the inside of the volume because of the dimensions of the sectionals of the ventilation circuits. The time constant with which pressure variations are transmitted to the inside of air-conditioned or ventilated vehicles is thus often small, because of the very characteristics of the ventilation circuits.
By way of example, it is known that when a vehicle moving through the atmosphere passes close to an obstacle (a tunnel entrance, a bridge, various objects in the vicinity of the track or road, etc), or when going past another vehicle, the field of aerodynamic velocities that the vehicle conveys with it is deformed.
Such a phenomenon results in rapid variations in pressure on the external walls of the vehicle.
The higher the speed of the vehicle, the more pronounced this effect is, since the pressures are approximately proportional to the square of the speed.
In particular, when very high speed trains enter tunnels or cuttings, rapid variations in pressure occur on the external walls of the rail cars. These sudden variations propagate through the confined space constituted by the tunnel or by the cutting, and they are known as "pressure waves".
Furthermore, these pressure waves, which propagate through the external confined space, are reflected at its ends, thereby creating return pressure waves that are as large as the direct waves.
These variations are passed rapidly on to the insides of the vehicles mainly via the ventilation or air-conditioning circuits, which is very unpleasant for the passengers.
It is thus common to be subjected to pressure variations that can reach 2,000 Pascals in less than one second.
The invention is also applicable to any volume ventilated by means of an air intake circuit and of an air extraction circuit, the circuits being in contact with the external pressure, and regardless of whether or not the volume leaks.
By way of example, the volume may communicate with the outside other than via the above-defined circuits, e.g. via openings, permeable gaskets, doors or windows that are not completely sealed, W.C. vents, etc.
Variations in external pressure are generally transmitted very rapidly, almost instantaneously, to the volume via leaks but above all via the ventilation circuits which are often very permeable to such variations.
Such variations in pressure are generally not attenuated very much, and they can be physiologically unpleasant, as in very high speed trains.
The installations in question, whether they be stationary volumes or vehicles, are such that it is not possible to shut off the ventilation circuits for long periods during external pressure variations since a minimum level of air renewal must be provided
Similarly, it is not possible, for technological reasons, to achieve perfect sealing between the inside and the outside of the volume.